That somewhat depends on how the list develops ÂÂ
I would say that the AX-1-0 pair is going to be very popular while its the only TK outside the Manta.
Can't see many times you'd need more than one Orca (unless the not-being-mentioned-fish stays/comes back).
Barracudas a pair will be necessary, most armies need interceptors, you might use two formations but realatively rarely (though if the list goes more air heavy that might change).
The current Tigershark isn't that exciting, if it gets drones drop back (50/50 at most) it might be worth a look.  There was suggestions that the above-unmentioned-fish might become a Tigershark variant, if so it might be worth a look, but so far as I am aware there have been no stats posted for it yet (E&C's list has a Orca-bomber-thing that might be similar).
I would say at least 6 each of Piranhas and Tetras.  Again it depends on how the list goes.  In 5.1 I was using a mix of 15, and as the points go up I used more Piranhas (more big AT targets as points go up).
